Challenges in construction of HS2 Diaphragm Wall shafts and Portal (UK)-Paper

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The HS2 C1 contract required four circular diaphragm-wall shafts, 18.16 m in internal diameter and up to 78.3 m deep, to support tunnel ventilation and intervention works through challenging chalk geology.<br /><br />Major construction risks included fractured and weathered chalk, dissolution features and low groundwater levels, which caused substantial bentonite loss during excavation of the first shaft. Pre-treatment fissure grouting was therefore carried out at subsequent sites, with post-treatment grouting around tunnel break-in/break-out blocks, adits and base plugs. The shafts also required tight verticality tolerances and reinforced “soft eyes” for the tunnel-boring machines.<br /><br />Chalk fines produced during cutting, together with concrete fines from milled panel joints, rapidly contaminated the bentonite. A bespoke gravity-fed recycling system and two centrifuges enabled mud density to be managed and bentonite reused, greatly reducing waste: at Chalfont St Peter, 6,300 m³ was excavated while only 400 m³ of mud was disposed of. Concrete working platforms improved equipment efficiency and avoided the maintenance problems experienced with the initial aggregate platform.<br /><br />Reinforcement-cage design, detailing, fabrication and delivery were significant programme constraints, particularly given limited UK supplier capacity. Cage configurations were revised to improve installation, and later shafts benefited from alternative suppliers. Concrete delivery rates, spacer exposure and congested coupler arrangements also required attention during construction.<br /><br />The four shafts were successfully completed and incorporated into the permanent works. Key lessons include involving specialist contractors early in reinforcement design, planning bentonite treatment and site layouts before mobilisation, and undertaking pre-construction testing to anticipate grout requirements and control fluid losses from the outset.

Keywords

HS2

diaphragm-wall shafts

chalk geology

bentonite recycling

fissure grouting

tunnel ventilation

construction risks

reinforcement cages

TBM soft eyes

groundwater loss